If you're hosting across multiple geographic locations, DNS failover is one of the simplest paths to high availability. The concept: publish multiple A records or use a DNS provider that health-checks your endpoints and removes unhealthy ones from rotation.
How it works:
- Configure two (or more) servers in different locations, each capable of serving the application
- Set up DNS with a health check monitor pointing at each server
- When a health check fails, the DNS provider stops returning that server's IP in responses
- Traffic automatically routes to the surviving server(s)
Caveats:
- TTL matters. If your DNS TTL is 3600 seconds (1 hour), some clients will keep hitting the dead server for up to an hour. Use low TTLs (60-300 seconds) for failover records
- DNS caching is unpredictable. Some ISPs and resolvers ignore TTLs entirely
- Session state: If users have sessions on Server A and fail over to Server B, those sessions are lost unless you use shared session storage
Modern approach: Load balancers (like Cloudflare, AWS ALB, or Azure Front Door) handle this more gracefully than raw DNS failover. But understanding the DNS-level mechanics is still fundamental.